Olefins are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefins such as ethylene and/or propylene from a variety of hydrocarbon feedstock. Ethylene and propylene are important commodity petrochemicals useful in many processes for making plastics and other chemical compounds. Ethylene is used to make various polyethylene plastics, and in making other chemicals such as vinyl chloride, ethylene oxide, ethylbenzene and alcohol. Propylene is used to make various polypropylene plastics, and in making other chemicals such as acrylonitrile and propylene oxide.
The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefins. This process is referred to as the oxygenate-to-olefin process. The preferred oxygenate for light olefin production is methanol. The process of converting methanol to olefins is called the methanol-to-olefins process.
There are numerous technologies available for producing oxygenates, and particularly methanol, including fermentation or reaction of synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials including coal, recycled plastics, municipal waste or any other organic material. The most common process for producing methanol is a two-step process of converting natural gas to synthesis gas. Then, synthesis gas is converted to methanol.
Generally, the production of synthesis gas involves a combustion reaction of natural gas, mostly methane, and an oxygen source into hydrogen, carbon monoxide and/or carbon dioxide. Synthesis gas production processes are well known, and include conventional steam reforming, autothermal reforming or a combination thereof.
Synthesis gas is then processed into methanol. Specifically, the components of synthesis gas (i.e., hydrogen, carbon monoxide and/or carbon dioxide) are catalytically reacted in a methanol reactor in the presence of a heterogeneous catalyst. For example, in one process, methanol is produced using a copper/zinc oxide catalyst in a water-cooled tubular methanol reactor.
The methanol is then converted to olefins in a methanol-to-olefins process. The methanol-to-olefins reaction is highly exothermic and produces a large amount of water. Water often comprises more than one half of the total weight of the effluent stream. Consequently, the water must be removed by condensation in a quench device to isolate the olefin product.
U.S. Pat. No. 6,121,504 describes a quench apparatus for an oxygenate to olefins process as well as a process for using a quench apparatus. The process removes water from the effluent stream as well as some oxygenate feedstock such as methanol. When the water is quenched, a portion of the water is cooled and recycled to the quench tower as quench medium. Additionally, the quench bottoms stream is passed through heat exchangers as a heat source for the methanol feed. Otherwise, water is sent to wastewater treatment.
U.S. Pat. No. 6,403,854 describes a two stage solids wash and quench for use with the oxygenate conversion process where catalyst fines are removed from the effluent stream through a first quench stage. Water and methanol is removed from the effluent stream in a second quench stage. The quench bottoms from the first quench stage is withdrawn as an aqueous waste stream or drag stream and is sent to a water treatment zone. A portion of the water from the quench bottoms of the second stage is cooled and reused as a quench medium. Otherwise, water is sent to wastewater treatment.
It would be advantageous, particularly in arid climates, to be able to find a better use for byproduct water from a methanol to olefins plant than to dispose of it in wastewater treatment. The present invention satisfies these and other needs.